Photodetectors, and one or two dimensional photodetector arrays comprising a compound semiconductor system having sensitivity in the near ultraviolet and the infrared regions that silicon detectors can not cover have broad demand in fields such as sensing devices for optical communications, spectroscopic systems, or as infrared cameras for medical treatment, disaster prevention, industrial inspection and others.
Especially in the recent years, development of a highly sensitive photodetector is strongly demanded for the infrared region where basic absorption bands exist in the environmental gases such as carbon monoxide, carbon dioxide, nitrous oxide and methane. The infrared detector in this wavelength band becomes important more and more as applications, such as air pollution surveillance, measures for global warming, disaster prevention, and reducing fuel consumption by combustion gas monitoring. Although infrared detectors made with an HgCdTe system have been manufactured for these applications, this material is a source of pollution so that the quantity of production and its usage have been regulated. Therefore, the infrared detector with a safer material is in demand. Also, infrared detectors using narrow gap semiconductors such as an InSb system have shown large dark current caused by thermal excitation or surface leak, which is the main reason for the dark current noise that limits the detection sensitivity. Therefore, in the application which requires high sensitivity, the device needs to be cooled less than −120 degrees Centigrade with liquid nitrogen or large size equipment such as a mechanical cooler in order to reduce the dark current.
Various contrivances have been made in order to suppress the dark current in a narrow gap semiconductor. As indicated by the well-known SRH (Shockley-Read-Hall) statistics, electron-hole pairs are generated when the product of electron and hole concentration are smaller than that of a thermal equilibrium state, and electron-hole pairs are recombined when the product is larger than that of a thermal equilibrium state. Moreover, in the case of a depletion state, a semiconductor of a narrow band gap exhibits higher carrier generation speed due to its large intrinsic carrier concentration. Therefore, in order to control dark current in the narrow band gap semiconductor for middle-infrared detection, it is important to make the depletion region small wherein electron-hole pairs are generated.
As the following document 1 discloses, an antimony system middle-infrared photodetector has so-called nBn structure, where a semiconductor layer with a large band gap is inserted between a photo-absorption layer of an n-type semiconductor with a narrow band gap and an n-type contact layer with the same narrow band gap in order to control dark current.
Document 1: S. Maimon and G. W. Wicks and “nBn detector, an infrared detector with reduced dark current and higher operating temperature” APPLIED PHYSICS LETTERS vol. 89, October 2006, p. 151109.”
In this structure, the dark current is suppressed as the current induced by electron majority carriers is blocked with the barrier formed in the conduction band of the larger band gap layer, and valence band barriers of all semiconductor which constitute a photodiode (PD) can be eliminated and the band offset in the valence band can be made almost flat so that only the photo excited hole current is effectively retrieved as a photo-induced current.
Also, document 2 shows the structure intending to detect infrared efficiently even in 4 μm wavelength region at room temperature, where a large band gap such as InAlAsSb is adopted for the surface barrier layer, and the discontinuity of a valence band in the boundary with the photo-absorption layers such as InAsSb is eliminated so that holes generated by light irradiation are captured efficiently.
Document 2: H. Shao, W. Li, A. Torfi, D. Moscicka, and W. I. Wang, “Room-Temperature InAsSb Photovoltaic Detectors for Mid-Infrared Applications”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, No. 16, August 15, and (2006) p. 1756-1758.
In addition, the inventors' separate research has revealed that the dark current of a mesa type photodiode with excellent isolation characteristics equivalent to a planar type can be made, where the mesa surface has the same electric conduction type and the pn junction formed with a narrow band gap material is not exposed to the mesa surface.
These photodiodes are simple in the operation mechanism and excellent in quantitative measurement, however, only single electron-hole pair is generated per photon at maximum, thus, the current output is too small to detect very weak light and there is a problem that the detectivity is limited by the noise characteristics of an electric amplifier connected to the photodiode. Therefore, a phototransistor having internal amplification function has been developed as a compound semiconductor photodetector. The hetero-junction bipolar phototransistors (HPT), which amplifies photocurrent by changing the potential of the base with the holes generated from the photon, has less noise than an avalanche photodiode (APD) having a similar amplifying effect of photocurrent and has been researched and developed continuously from the 1980s until now as a highly sensitive detector.
A phototransistor usually has its base-collector junction to serve as a photodiode, and by injecting photocarriers generated in the collector region into the base, the base is biased to the forward direction as in a bipolar transistor. Therefore, the characteristics of HPT are determined by the characteristics of a photodiode (PD) and a hetero-bipolar transistor (HBT), and by those electric connection methods so that optimally designing these structural elements is necessary.
When the HPT structure is implemented by the multiple layer epitaxial growth, emitter base layers or base collector layers are made into a mesa structure for isolation. According to the inventors' research findings described previously, enhancement of the current gain of the transistor and the suppression of dark current are simultaneously achieved by turning the mesa surface regions of each layer into common electric conduction type. Moreover, according to the inventors' other research findings, the dark current of the PD section can be suppressed even in a stacked DH (Double Hetero) type HBT and a planar type PD made by selective impurity diffusion.
In a heterojunction bipolar transistor (HBT) used as an electron device, an emitter material can be made so as to have a larger band gap relatively to a base material, and to make the energy level of a valence band of the emitter lower than that of the base. In other words, leakage of holes from the base to the emitter can be blocked as holes encounter the electrostatic potential barrier in the emitter interface, which in turn, produces large current gain. Furthermore, in order to improve the breakdown voltage of a transistor, the DH type HBT using a material having a comparatively wide band gap as a collector is also being developed.
In order to extend the sensitive wavelength range of a phototransistor, it is common to make the energy band gap of the collector small so as to correspond to the wavelength band to be used. However, the problem occurs that the breakdown voltage between the emitter and the collector drops because the electric field concentrates on the base collector junction. In general, the breakdown voltage of semiconductor material drops as its band gap is narrowed so that it is disadvantageous to use a narrow gap material having an absorption edge in a long wavelength region as a collector.
On the other hand, in a DH type HBT, which is advantageous from the viewpoint of breakdown voltage, the electron transfer from the base to the collector is obstructed since the conduction band level of the collector becomes higher than that of the base. Moreover, when the photo-absorption layer corresponding to the detecting wavelength is formed under the collector layer having a relatively large energy band gap, the energy level of the valence band of the photo-absorption layer becomes lower than that of the collector, and a path reaching the base from the photo-absorption layer through photo-generated holes is blocked. Therefore, a DH type HBT is not suitable as a phototransistor for detecting very weak light.
In the case of a phototransistor, having a high current amplification factor is required in order to amplify the photo-induced current of the sub pA level corresponding to absorbed photons even for the very small base current. Moreover, in a photodetector, cooling a device is generally performed in order to control the dark current. Under the conditions of such very small bias current and a low-temperature operation, a new design of a band profile is essential because the small discontinuity in the band profile obstructs the transfer of electrons and holes, which was not a concern for a conventional HBT intending to amplify large current.
In III-V compound semiconductors, it is possible to design potentials of the conduction band and the valence band independently by arranging the composition ratio of the constitutional elements. Especially, a compound semiconductor containing antimony has a tendency of a conduction-band level to become high as its electron affinity is small. For example, it is possible to design the conduction band of a base layer higher than an InP collector and also to make the valence band of a base layer higher than an InP emitter, by replacing a GaAsSb lattice-matched to InP with an InGaAs base for the InP system DH-HBT. See the following documents 3 and 4 concerning this point.
Document 3: R. Bhat, W-P. Hong, C. Caneau, M. A. Koza, C-K. Nguyen, and S. Goswami and “InP/GaAsSb/InP and InP/GaAsSb/InGaAsP double heterojunction bipolar transistors with a carbon-doped base grown by organometallic chemical vapor deposition”, Applied Physics Letters vol. 68 (7), February 1996 pp. 985-987.
Document 4: C. R. Bolognesi, N. Matine, Martin W. Dvorak, P. Yeo, X. G. Xu, and Simon P. Watkins, “InP/GaAsSb/InP Double HBTs: A New Alternative for InP-Based DHBTs”, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, No. 11, November 2001 pp. 2361-2639.
In the following document 5, it is proposed to make electron transfer smooth with an antimony system compound semiconductor, by lowering step-wise conduction bands of an emitter, a base, and a collector, and by matching the potential of each boundary.
Document 5: U.S. Patent Publication No. 2006/0065952
In InGaAs formed on an InP substrate, the sensitive wavelength of about 2.2 μm is the practical limit even if the In composition is increased and a certain amount of lattice mismatch is allowed. When a photo-absorption layer is formed with antimony system semiconductors, such as GaSb, InGaSb, and InAsSb, the photodetection wavelength can be extended to about 2.8 μm with an InGaSb photo-absorption layer, and about 5 μm with an InAsSb photo-absorption layer. Even in a HPT, the design concept similar with an electron device HBT is applied to make small the band offset of the conduction band between an emitter and a bases, and also to make large the band offset of a valence band in order to obtain a large amplification action. As for a middle-infrared photodetector, as described in the following documents 6 and 7, an AlGaAsSb/InGaAsSb system HPT formed on a GaSb substrate produces gain from 500 A/W to 1000 A/W at near room temperature. However, since the collector serves as the photo-absorption layer, the detection wavelength can be up to 2.5 μm band at most.
Document 6: M. Nurul Abedin, Tamer F. Refaat, Oleg V. Sulima, and Upendra N. Singh, “AlGaAsSb—InGaAsSb HPTs With High Optical Gain and Wide Dynamic Range”, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 51, No. 12, December 2004, p. 2013-2018.
Document 7: Tamer F. Refaat, M. Nurul Abedin, Oleg V. Sulima, Syed Ismail, and Upendra N. Singh, “2.4-μm-Cutoff AlGaAsSb/InGaAsSb Phototransistors for Shortwave-IR Applications”, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 54, No. 11, November 2007 p. 2837-2842.
Since the above-mentioned antimony system compound semiconductor is a high mobility material and has attracted attention as an ultra high-speed electron device application for the terahertz band. In an HBT, the band gap of a base layer determines theoretical amplitude so that the AlSb/GaSb/InAs system material having a large band offset and containing high mobility narrowband-gap materials has been drawn attention since. In particular, replacing the base layer of AlGaAs/GaAs system or InP/InGaAs system with GaAsSb to remove the notch of a conduction band in the HBTs (see the above-mentioned Documents 3 and 4), and also in the inventors' another experiments, using an InGaSb as a base layer and InAlAsSb as an emitter layer and collector layer in order to lower a theoretical amplitude than in an InGaAs/InP system HBT has been attempted. In this case, particular attention is also required for the composition selection of a collector material to remove the notch in the boundary of conduction band of a base collector in order to reduce theoretical amplitude and to lower power consumption.
However, by using a narrow band gap semiconductor as a base layer, the recombination effect caused by the Auger effect increases and the breakdown voltage drops. In the conventionally disclosed Document group, there is no disclosure to particularly note when an HPT is operated as a middle-infrared photodetector.
Since an infrared phototransistor has the structure where the transistor region is stacked on the photodiode region, it is necessary to meet the following 3 conditions simultaneously: (a) control of the dark current to improve a photodiode detectivity, (b) transfer of the photo-generated electric charge, and (c) reservation of the transistor current amplification effect.
That is, firstly, in order to extend the photodetection wavelength to a long wavelength band in a phototransistor, (1) it is necessary to control the dark current of the PD section being used as a photo-absorption layer. For that purpose, it is necessary to have such a structure where the pn junction of a narrowband gap semiconductor does not expose at the surface of a PD. Moreover, in order to control the dark current generated inside a bulk, it is also effective to form a potential barrier in a conduction band to block the electron current component.
Secondly, (2) the potential barrier in the valence band through a photo-absorption layer to a base layer is removed as oppose to the photo-generated hole, the charge generated in the PD section is duly transported to the transistor region. In other words, it is necessary to make a barrier-free profile.
Furthermore, (3) in a transistor region, it is necessary to have such a structure where a large current gain can be obtained corresponding to very small base current. For that purpose, (3-1) the surface recombination is suppressed in an end surface of the emitter base junction, (3-2) the potential barrier of sufficient height is formed for a valence band in the emitter base junction, (3-3) there is no potential barrier which obstructs the electron transfer in the conduction band from the base to the collector, in other words, it is necessary to make a barrier-free profile. (3-4) The ON voltage between the emitter and base should not be excessive for the operating temperature range of the device. (3-5) Moreover, in the practical environment, it is required that the reverse breakdown voltage should be large enough.
When overlapping a collector layer with a photo-absorption layer, it is necessary to narrow the band gap of a collector layer corresponding to a detecting wavelength. On the other hand, in order to increase the reverse breakdown voltage between a base and collector, it is necessary to expand band gaps that form the material for a base layer and a collector layer to some extent. Therefore, in order to make an HPT of which the breakdown voltage is large enough in the middle-infrared area, it is necessary to separate the photo-absorption layer and the collector layer, and to use a comparatively large band gap material for a collector layer.
However, the conventional DH type HBT which uses a comparatively large band gap material for a collector layer has an insufficient current gain for a very small current, and is not suitable for application to a HPT. Also, it is necessary to transfer the holes from the photo-absorption layer to the collector interface smoothly.
Also, when the conduction band of the collector layer is higher than the base, the hetero barrier formed in the collector base boundary reduces the electron injection efficiency. The barrier of the conduction band generated in such a collector interface obstructs the electron transfer. Especially, it reduces the amplification action for weak current during the low-temperature operation.